Increased use of surgical techniques to correct vision problems has resulted in an increased need for data relating to the topography of the cornea of the eye. Deformations in the cornea of the eye are mainly responsible for vision problems experienced by patients. The shape of the patient's cornea is a significant factor to such eye diseases such as myopia. An eye with a perfect vision has a near spherical cornea so that incident light is diffracted inward towards a focal point within the eye. Variations in the shape of the cornea can result in light not being diffracted into the focal point of the eye thereby producing vision problems for the patient. These eye problems are typically corrected by positioning a lens in front of the eye, which is configured to be able to correct for the deformations in the patient's cornea which are causing the eye problem.
In the past, the correction needed by a particular patient was determined by positioning a series of lenses in front of the patient's eye until their vision improved. However, as analytic techniques and instrumentations have become more sophisticated, mapping of the cornea to obtain the overall contour of the cornea has become more common. Corneal topography data provides a treating physician with information as to the localized radius of curvature of a particular cornea. This allows the treating physician to more accurately select contact lenses and it also greatly aids the treating physician in correcting eye deformations through surgical techniques.
Recently, the use of surgical techniques to correct eye problems such as myopia, have become more common. Techniques such as radial keratotomy and other well known techniques require that the treating physician have detailed information as to the configuration of the patient's cornea. With this information, the treating physician can then cut, abate, or otherwise change the outer surface of the cornea at various locations to alter the overall shape of the cornea to thereby correct the patient's vision. In fact, these techniques have become significantly advanced so that treating physicians are able to correct significant nearsightedness or far-sightedness to near perfect vision. The treating physician needs detailed corneal topography information to perform these surgical techniques and also to fit contact lenses in specific situations. As a consequence, corneal topography systems have been developed which provide detailed information about the topography of the outer surface of a patient's cornea.
Corneal topography systems generally project into the patient's eye a placido image which is an image of a plurality of concentric rings or mires. The image of these rings is reflected off of the patient's cornea and is then captured using a camera. Thus, the camera contains a two-dimensional image of the rings being reflected off of the patient's three-dimensional cornea. The position of the reflected rings in the captured image can then be used to calculate the curvature of the patient's eye.
Specifically, it is assumed that a cornea having perfect vision will be generally uniformly spherical. If the placido image was reflected off of a perfectly spherical surface, the reflected rings would appear on a two-dimensional image as a plurality of concentric rings with the two-dimensional location of the rings being related to the curvature of the spherical surface. If, however, the patient's cornea is not perfectly spherical, the positions of the plurality of rings in the resulting reflected image are generally displaced from the corresponding position of the rings that is reflected off of the perfect sphere. A comparison of the position between the image reflected off of the patient's cornea and a corresponding perfect sphere will permit the determination of the deviation of the patient's cornea from a perfect sphere. In this manner, the radius of curvature of the patient's cornea at locations over the entire surface area of the patient's eye can be calculated thereby providing the topography of the patient's cornea.
A placido projector is typically used to project a placido image. The placido projector was first used in 1880 by a Portuguese ophthalmologist named Antonio Placido who used a painted disk (Placido's disk) of alternating black and white rings to project contour lines onto the cornea. Conventional placido projectors comprise a cone of translucent material. The inner surface of the placido projector is coated with a plurality of concentric opaque rings. FIG. 1 shows a placido projector made of a translucent material, such as plastic, which has a plurality of concentric rings painted on its inner surface. FIG. 2 shows the outer surface of the placido projector that is generally frusto conical in shape. A light source such as an EL panel is positioned immediately adjacent to the outer surface of the placido projector so as to uniformly illuminate the placido projector to thereby produce the placido image that is to be reflected off of the patient's cornea.
FIGS. 3 and 4 illustrate the configuration of the light source. The light source is comprised of an EL panel 300 that is cut into a half circle that can be folded together to form the frusto conical shape 400 shown in FIG. 4. As discussed before, the outer surface of the placido projector is also frusto conical. Hence the EL 300 panel includes a cut-out 304 that is sized so that when the EL panel 300 is folded into the frusto conical shape 400, an opening 404 is formed. The opening 404 corresponds to an opening of the placido projector to allow the reflected image of the placido to be received by a camera.
Thus, existing placido projectors comprise a cone of translucent material, the inner surface of which is coated with a plurality of concentric opaque rings. A light source is positioned immediately adjacent to the outer surface of the translucent cone to illuminate the placido projector to thereby produce the placido image that is to be reflected off of the patient's cornea.
While concentric ring images on placido projectors can be produced on a machinist lathe, the production requires a significant investment in tooling and fixtures to hold a uniquely shaped part in a conventional machinist tool, and the process is time intensive. Concentric ring images may be produced by molding operations, which also requires a machining process to add and/or remove opaque material. Also, certain non-circular patterns cannot be created using machining or molding operations.